How to trigger Mysql parallel query function to start?

Please help,how to trigger Mysql parallel query function to start? MySQL version is 8.0.19.

mysql> show variables like 'innodb_parallel_read_threads'; +------------------------------+-------+ | Variable_name                | Value | +------------------------------+-------+ | innodb_parallel_read_threads | 4     | +------------------------------+-------+ 1 row in set (0.00 sec) 

Judging from the execution plan, although the cost value is high, it does not trigger parallel queries.

mysql> explain analyze SELECT count(a.name) FROM b,a WHERE b.id = a.id AND a.id < 10000\G *************************** 1. row *************************** EXPLAIN: -> Aggregate: count(a.`name`)  (actual time=79199.970..79199.971 rows=1 loops=1)     -> Inner hash join (b.id = a.id)  (cost=29804029261222.55 rows=29803521726988) (actual time=52129.791..79198.823 rows=9999 loops=1)         -> Table scan on b  (cost=31.81 rows=18758239) (actual time=0.385..19630.712 rows=20000000 loops=1)         -> Hash             -> Filter: (a.id < 10000)  (cost=4909601.51 rows=15888229) (actual time=2.266..52117.583 rows=9999 loops=1)                 -> Table scan on a  (cost=4909601.51 rows=47669458) (actual time=2.262..48985.413 rows=50000000 loops=1)  1 row in set (1 min 19.25 sec)